Available in Apache 2.3 and up, on Solaris 10 and
OpenSolaris platforms

Summary

This module enables different Virtual Hosts to run with different
Unix User and Group IDs, and with different
Solaris Privileges. In particular, it offers a solution to the
problem of privilege separation between different Virtual Hosts, first
promised by the abandoned perchild MPM. It also offers other security
enhancements.

Unlike perchild, mod_privileges
is not itself an MPM. It works within a processing model to
set privileges and User/Group per request in a running process.
It is therefore not compatible with a threaded MPM, and will refuse
to run under one.

mod_privileges raises security issues similar to
those of suexec. But unlike suexec,
it applies not only to CGI programs but to the entire request processing
cycle, including in-process applications and subprocesses.
It is ideally suited to running PHP applications under mod_php,
which is also incompatible with threaded MPMs. It is also well-suited
to other in-process scripting applications such as mod_perl,
mod_python, and mod_ruby, and to
applications implemented in C as apache modules where privilege
separation is an issue.

Directives

Topics

mod_privileges introduces new security concerns
in situations where untrusted code may be run
within the webserver process. This applies to
untrusted modules, and scripts running under modules such as
mod_php or mod_perl. Scripts running externally (e.g. as CGI
or in an appserver behind mod_proxy or mod_jk) are NOT affected.

The basic security concerns with mod_privileges are:

Running as a system user introduces the same security issues
as mod_suexec, and near-equivalents such as cgiwrap and suphp.

A privileges-aware malicious user extension (module or script)
could escalate its privileges to anything available to the
httpd process in any virtual host. This introduces new risks
if (and only if) mod_privileges is compiled with the
BIG_SECURITY_HOLE option.

A privileges-aware malicious user extension (module or script)
could escalate privileges to set its user ID to another system
user (and/or group).

The PrivilegesMode directive allows you to
select either FAST or SECURE mode. You can
mix modes, using FAST mode for trusted users and
fully-audited code paths, while imposing SECURE mode where an
untrusted user has scope to introduce code.

Before describing the modes, we should also introduce the target
use cases: Benign vs Hostile. In a benign situation, you want to
separate users for their convenience, and protect them and the server
against the risks posed by honest mistakes, but you trust your users
are not deliberately subverting system security. In a hostile
situation - e.g. commercial hosting - you may have users deliberately
attacking the system or each other.

FAST mode

In FAST mode, requests are run in-process with the
selected uid/gid and privileges, so the overhead is negligible.
This is suitable for benign situations, but is not secure against an
attacker escalating privileges with an in-process module or script.

SECURE mode

A request in SECURE mode forks a subprocess, which
then drops privileges. This is a very similar case to running CGI
with suexec, but for the entire request cycle, and with the benefit
of fine-grained control of privileges.

You can select different PrivilegesModes for
each virtual host, and even in a directory context within a virtual
host. FAST mode is appropriate where the user(s) are
trusted and/or have no privilege to load in-process code.
SECURE mode is appropriate to cases where untrusted code
might be run in-process. However, even in SECURE mode,
there is no protection against a malicious user who is able to
introduce privileges-aware code running before the start of the
request-processing cycle.

Available on Solaris 10 and OpenSolaris with
non-threaded MPMs (prefork or custom MPM).

This server-wide directive determines whether Apache will run with
the privileges required to run
dtrace.
Note that DTracePrivileges On will not in itself
activate DTrace, but DTracePrivileges Off will prevent
it working.

Available on Solaris 10 and OpenSolaris with
non-threaded MPMs (prefork or custom MPM).

This directive trades off performance vs security against
malicious, privileges-aware code. In SECURE mode, each request
runs in a secure subprocess, incurring a substantial performance penalty.
In FAST mode, the server is not protected against escalation
of privileges as discussed above.

This directive differs slightly between a <Directory>
context (including equivalents such as Location/Files/If) and a
top-level or <VirtualHost>.

At top-level, it sets a default that will be inherited by virtualhosts.
In a virtual host, FAST or SECURE mode acts on the entire
HTTP request, and any settings in a <Directory>
context will be ignored. A third pseudo-mode
SELECTIVE defers the choice of FAST vs SECURE to directives in a
<Directory> context.

In a <Directory> context, it is applicable only
where SELECTIVE mode was set for the VirtualHost. Only
FAST or SECURE can be set in this context (SELECTIVE would be
meaningless).

Warning

Where SELECTIVE mode is selected for a virtual host, the activation
of privileges must be deferred until after the mapping
phase of request processing has determined what
<Directory> context applies to the request.
This might give an attacker opportunities to introduce
code through a RewriteMap
running at top-level or <VirtualHost> context
before privileges have been dropped and userid/gid set.

Available on Solaris 10 and OpenSolaris with
non-threaded MPMs (prefork or custom MPM).

Determines whether the virtual host is allowed to run fork and exec,
the privileges required to run subprocesses. If this is set to
Off the virtualhost is denied the privileges and will not
be able to run traditional CGI programs or scripts under the traditional
mod_cgi, nor similar external programs such as those
created by mod_ext_filter or
RewriteMapprog.
Note that it does not prevent CGI programs running under alternative
process and security models such as mod_fcgid, which is a recommended solution in Solaris.

If set to On or Secure, the virtual host
is permitted to run external programs and scripts as above.
Setting VHostCGIModeSecure has
the effect of denying privileges to the subprocesses, as described
for VHostSecure.

Available on Solaris 10 and OpenSolaris with
non-threaded MPMs (prefork or custom MPM)
and when mod_privileges is compiled with the
BIG_SECURITY_HOLE compile-time option.

VHostCGIPrivs can be used to assign arbitrary privileges to subprocesses created by a virtual host, as discussed
under VHostCGIMode. Each privilege-name
is the name of a Solaris privilege, such as file_setid
or sys_nfs.

A privilege-name may optionally be prefixed by
+ or -, which will respectively allow or deny a privilege.
If used with neither + nor -, all privileges otherwise assigned
to the virtualhost will be denied. You can use this to override
any of the default sets and construct your own privilege set.

Security

This directive can open huge security holes in apache subprocesses,
up to and including running them with root-level powers. Do not
use it unless you fully understand what you are doing!

Available on Solaris 10 and OpenSolaris with
non-threaded MPMs (prefork or custom MPM).

The VHostGroup directive sets the Unix group
under which the server will process requests to a virtualhost.
The group is set before the request is processed and reset afterwards
using Solaris Privileges. Since the setting applies to the
process, this is not compatible with threaded MPMs.

Unix-group is one of:

A group name

Refers to the given group by name.

# followed by a group number.

Refers to a group by its number.

Security

This directive cannot be used to run apache as root!
Nevertheless, it opens potential security issues similar to
those discussed in the suexec
documentation.

Available on Solaris 10 and OpenSolaris with
non-threaded MPMs (prefork or custom MPM)
and when mod_privileges is compiled with the
BIG_SECURITY_HOLE compile-time option.

VHostPrivs can be used to assign arbitrary privileges to a virtual host. Each privilege-name
is the name of a Solaris privilege, such as file_setid
or sys_nfs.

A privilege-name may optionally be prefixed by
+ or -, which will respectively allow or deny a privilege.
If used with neither + nor -, all privileges otherwise assigned
to the virtualhost will be denied. You can use this to override
any of the default sets and construct your own privilege set.

Security

This directive can open huge security holes in apache, up to
and including running requests with root-level powers. Do not
use it unless you fully understand what you are doing!

Available on Solaris 10 and OpenSolaris with
non-threaded MPMs (prefork or custom MPM).

Determines whether the virtual host processes requests with
security enhanced by removal of Privileges that are rarely needed in a webserver, but which are
available by default to a normal Unix user and may therefore
be required by modules and applications. It is recommended that
you retain the default (On) unless it prevents an application running.
Since the setting applies to the process, this is not
compatible with threaded MPMs.

Note

If VHostSecure prevents an application
running, this may be a warning sign that the application should be
reviewed for security.

Available on Solaris 10 and OpenSolaris with
non-threaded MPMs (prefork or custom MPM).

The VHostUser directive sets the Unix userid
under which the server will process requests to a virtualhost.
The userid is set before the request is processed and reset afterwards
using Solaris Privileges. Since the setting applies to the
process, this is not compatible with threaded MPMs.

Unix-userid is one of:

A username

Refers to the given user by name.

# followed by a user number.

Refers to a user by its number.

Security

This directive cannot be used to run apache as root!
Nevertheless, it opens potential security issues similar to
those discussed in the suexec
documentation.

See also

Notice:This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our mailing lists.